Support nested coreschema in CoreAPI compat layer

[khomyakov42]

class MyFilterBackend(BaseFilterBackend):
   def get_schema_fields(self, view):
      return [coreapi.Field(
         name="values"
         required=False, 
         schema=coreschema.Array(items=coreschema.Integer(), unique_items=True), 
         location='query'
      )]

Result:

[axnsan12]

This is not currently supported. While it would be nice, the way to do it now is to create a FilterInspector.